*3.1. Structure and Properties of Sintered Ceramic Blanks*

Figure 9 shows the experimentally obtained data on the effect of various powder compositions (1) 80% (90α10β) + 20% TiN, (2) 90% (90α10β) + 10% TiN and (3) 80% (70α30β) + 20% TiN and spark plasma temperatures in the range of 1600–1750 ◦C on the basic physical and mechanical properties of sintered ceramic samples: percentage theoretical density (Figure 9a), hardness (Figure 9b), crack resistance (Figure 9c), and flexural strength (Figure 9d).

**Figure 9.** Dependences of the basic physical and mechanical properties of sintered ceramic samples made of various powder compositions such as 80% (90α10β) + 20% TiN (1), 90% (90α10β) + 10% TiN (2), and 80% (70α30β) + 20% TiN (3) from the spark plasma sintering temperature: (**a**) percentage theoretical density; (**b**) hardness; (**c**) crack resistance; and (**d**) flexural strength.

A general characteristic tendency of the temperature factor's influence at spark plasma sintering on the basic physical and mechanical properties is visible in Figure 9. The best values of the properties in all cases are achieved at a process temperature of 1700 ◦C. A further increase in temperature leads to a sharp decrease in the sintered material properties. The exception is hardness, which at 1750 ◦C remains at the same level for all specimens, Figure 9b. There is no doubt that a higher sintering temperature promotes recrystallization processes leading to rapid grain growth. The temperature of 1700 ◦C is a rational value for sintering all studied powder compositions based on SiAlON. Ceramics with a minimum number of pores are formed under these conditions judging by the fact that they have the maximum theoretical density (Figure 9a). It explains the maximum values at 1700 ◦C of crack resistance (Figure 9c) and flexural strength (Figure 9d).

Analysis of the basic physical and mechanical properties of the samples sintered from three versions of powder compositions shows that the best ratio of "theoretical density– hardness–crack resistance–flexural strength" in all cases had option (1): ceramic base α–β SiAlON 80 wt.% (α–SiAlON 90 wt.% + β–SiAlON 10 wt.%) + TiN 20 wt.% (composition code 80% (90α10β) + 20% TiN). The indicated samples at a temperature of 1700 ◦C have the following quantitative values of properties: theoretical density of 99.1%, hardness of 16.0 GPa, crack resistance of 5.3 MPa·m<sup>1</sup> <sup>2</sup> , and flexural strength of 550 MPa. It should be noted that the hardness of the experimental ceramics is in no way inferior to the industrially produced ceramics based on SiAlON, stabilized with Yb2O3 additives, but it is somewhat inferior to the best samples in terms of strength properties and crack resistance [30,37,39].

Summarizing the character of the experimentally obtained curves for various powder compositions (Figure 9), it can be concluded that the introduction of TiN particles in a volume of 10 wt.% during sintering (curve 2) does not provide an acceptable level of properties of ceramics based on α–β SiAlON (mainly the above refers to crack resistance). In contrast, the introduction of TiN into the powder composition in a volume of 20 wt.% (curve 1) provides the best combination of properties. In this case, the SiAlON ceramic base's rational composition is the following ratio between the phases: α–SiAlON 90 wt.% and β–SiAlON 10 wt.%. It was shown experimentally that an increase in the β-SiAlON content to 30 wt.% in a ceramic powder composition noticeably decreases the sintered material's strength characteristics (curve 3).

Table 5 shows the results of an elemental quantitative EDX analysis of an experimental sample of 80% (90α10β) + 20% TiN and the ceramic material, from which commercial ceramic end mills are made, intended for the processing of nickel alloys, taken in the current study as a standard. The data of an experimental sample of 80% (90α10β) + 20% TiN showed the best combination of physical and mechanical properties in studies. As follows from the presented data, the experimental ceramics contains the following content of elements: 37.4% Si, 19.1% Ti, and 12.6% Al. Commercial ceramic significantly differs in elemental composition and contains: 45.7% Si, 7.0% Yb, and 6.5% Al. Furthermore, we studied two fundamentally different types of ceramics based on SiAlON.

**Table 5.** Results of elemental quantitative EDX analysis of sintered ceramics based on SiAlON.


Figure 10 shows the results of microstructural analysis of fractures of two types of ceramics based on SiAlON, which demonstrate significant differences in the experimental sample 80% (90α10β) + 20% TiN (Figure 10a,c,e) and specimens of commercial ceramics (Figure 10b,d,f). It is natural since sintered ceramics' structure depends mainly on the type and number of sintering components and additives and the technological process parameters. Comparing scanning electron microscopy (SEM) images of the microstructures of the fracture surface of ceramic samples shows that both samples contain a significant amount of the α-modification of SiAlON as the dominant phase. This phase has high homogeneity and fineness for a specimen of commercial ceramics. Experimental ceramics are characterized by equiaxed and non-equiaxed grains with an average grain size of 3–5 μm, while commercial ceramics have a grain size of 1.5–3.5 μm. Besides, both samples have a certain content of the β-modification of SiAlON. This phase is more characteristic and noticeable in commercial ceramics since it has elongated (oblong) particles.

**Figure 10.** *Cont.*

**Figure 10.** SEM images of fracture surface microstructures of sintered ceramic samples based on SiAlON: (**a**) experimental sample 80% (90α10β) + 20% TiN, ×1000; (**b**) specimen of commercial ceramics, ×1000; (**c**) experimental sample 80% (90α10β) + 20% TiN, ×5000; (**d**) specimen of commercial ceramics, ×5000; (**e**) experimental sample 80% (90α10β) + 20% TiN, ×10,000; and (**f**) specimen of commercial ceramics, ×10,000.

The elements distribution maps along the samples' fracture were obtained using EDX analysis for the possibility of a qualitative assessment of the uniformity of distribution in ceramic samples of the main elements and Ti- and Yb-containing phases (Figure 11). It can be seen that the Ti-containing phase is fairly uniformly distributed over the material volume after sintering in the experimental sample of 80% (90α10β) + 20% TiN (Figure 11a–c). Element distribution maps in a specimen of commercial ceramics (Figure 11d–f) demonstrate that the Yb-containing phase is uniformly distributed throughout the material structure. It can be concluded that the Yb-containing phase (stabilizing phase) is formed along the grain boundaries of ceramics based on SiAlON, and it is this that inhibits their growth during sintering and ensures the formation of a finer-grained and uniform structure in comparison with samples from experimental ceramics 80% (90α10β) + 20% TiN (Figure 10) based on the data of authoritative researchers [37–40].

**Figure 11.** Distribution of chemical elements on the fracture surface of sintered ceramic material based on SiAlON samples, obtained by EDX analysis: (**a**) Si-content of experimental sample 80% (90α10β) + 20% TiN; (**b**) Al-content of experimental sample 80% (90α10β) + 20% TiN; (**c**) Ti-content of experimental sample 80% (90α10β) + 20% TiN; (**d**) Si-content of commercial ceramics specimen; (**e**) Al-content of commercial ceramics specimen; and (**f**) Yb-content of commercial ceramics specimen.
